Blender as a VSE - can I separate my editing into multiple scenes and render as single video?
In the end, I solved this by creating a final Scene (which I called 'All_Scenes') and then adding the other Scenes as Scene strips in the VSE (Header menu: Add >> Scene).  In the scene properties, I had to configure the rendering to output as mp4.

In the Scene properties, I had to ensure the 'Use Sequence' property was ticked.

I could then render the scene with:

 blender -b demo2016.blend -S All_Scenes -a